Designing Non-Deterministic Finite Automata (Set 4)

Prerequisite: Finite Automata Introduction In this article, we will see some designing of Non-Deterministic Finite Automata (NFA). Problem-1: Construction of a minimal NFA accepting a set of strings over {a, b} in which each string of the language contain ‘a’ as the substring. Explanation: The desired language will be like:

L1 = {ab, abba, abaa, ...........}

Here as we can see that each string of the above language contains ‘a’ as the substring. But the below language is not accepted by this NFA because some of the string of below language does not contain ‘a’ as the substring.

L2 = {bb, b, bbbb, .............}

The state transition diagram of the desired language will be like below: In the above NFA, the initial state ‘X’ on getting ‘a’ as the input it transits to a final state ‘Y’ and on getting ‘b’ as the input it remains in the state of itself. The final state ‘Y’ on getting either ‘a’ or ‘b’ as the input it remains in the state of itself. Refer for DFA of above NFA.

Transition Table :

In this table initial state is depicted by —> and final state is depicted by *.

STATES	INPUT (a)	INPUT (b)
—> X	Y*	X
Y*	Y*	Y*

Python implementation:

C++

#include <iostream>
#include <string>

// Function declarations
void stateX(const std::string& n);
void stateY(const std::string& n);

// Function implementations
void stateY(const std::string& n) {
    if (n.empty()) {
        std::cout << "string accepted" << std::endl;
    } else if (n[0] == 'a') {
        stateY(n.substr(1));
    } else if (n[0] == 'b') {
        stateY(n.substr(1));
    }
}

void stateX(const std::string& n) {
    if (n.empty()) {
        std::cout << "string not accepted" << std::endl;
    } else if (n[0] == 'a') {
        stateY(n.substr(1));
    } else if (n[0] == 'b') {
        stateX(n.substr(1));
    }
}

int main() {
    // Assumed string
    std::string assumedString = "bab";

    // Call stateX function to check the assumed string
    stateX(assumedString);

    return 0;
}

Python3

def stateX(n):
    #if length of n become 0
    #then print not accepted
    if(len(n)==0):
        print("string not accepted")

    else:
        #if at zero index
        #'a' found then call
        #stateY function
        if (n[0]=='a'):
            stateY(n[1:])

        #if at zero index
        #'b' then call
        #stateX function
        elif (n[0]=='b'):
            stateX(n[1:])

def stateY(n):
    #if length of n become 0
    #then print accepted
    if(len(n)==0):
        print("string accepted")

    else:
        #if at zero index
        #'a' found call
        #stateY function
        if (n[0]=='a'):
            stateY(n[1:])

        #if at zero index
        #'b' found call
        #stateY function
        elif (n[0]=='b'):
            stateY(n[1:])


#take input
n=input()

#call stateA function
#to check the input
stateX(n)

Problem-2: Construction of a minimal NFA accepting a set of strings over {a, b} in which each string of the language is not containing ‘a’ as the substring. Explanation: The desired language will be like:

L1 = {b, bb, bbbb, ...........}

Here as we can see that each string of the above language is not containing ‘a’ as the substring But the below language is not accepted by this NFA because some of the string of below language is containing ‘a’ as the substring.

L2 = {ab, aba, ababaab..............}

The state transition diagram of the desired language will be like below: In the above NFA, the initial and final state ‘Y’ on getting ‘b’ as the input it remains in the state of itself.

Transition Table :

In this table initial state is depicted by —> and final state is depicted by *.

STATES	INPUT (a)	INPUT (b)
—> Y *	Y*	Y*

 Implementation:

Java

import java.util.*;
import java.io.*;

class GFG
{
    public static void stateY(StringBuilder n)
    {
        // if length of n become 0
        // then print accepted
        if(n.length() == 0)
            System.out.println("string accepted");

        else
        {
            // if at zero index
            // 'a' found then
            // print not accepted
            if (n.charAt(0)=='a')
                System.out.println("String not accepted");

            // if at zero index
            // 'b' found call
            // stateY function
            else if(n.charAt(0)=='b')
            {
                n.deleteCharAt(0);
                stateY(n);
            }
        }
    }
    public static void main (String[] args)
    {
        Scanner sc = new Scanner(System.in);

        // take input
        String n = sc.next();
        StringBuilder b = new StringBuilder(n);

        // call stateY function
        // to check the input
        stateY(b);
    }
}

// This code is contributed by aakash_k.

Python3

def stateY(n):
    #if length of n become 0
    #then print accepted
    if(len(n)==0):
        print("string accepted")

    else:
        #if at zero index
        #'a' found then
        #print not accepted
        if (n[0]=='a'):
            print("String not accepted")

        #if at zero index
        #'b' found call
        #stateY function
        elif (n[0]=='b'):
            stateY(n[1:])


#take input
n=input()

#call stateY function
#to check the input
stateY(n)